1. Field of Invention
The present invention relates to hearing aids.
FIG. 1 of the accompanying drawings shows the frequency and intensity of distribution of speech sounds, which approximately occupy an intensity range of 25 dB. In normal working and social lives, peak speech levels of 55 to 85 dB are commonly encountered. Thus the total dynamic range of speech to be perceived is 85-(55-25) or 55 dB.
A large proportion of bearing-impaired people have tone audiograms in which the threshold of hearing is elevated and the threshold of discomfort remains the same as that for a healthy ear. Thus the dynamic range is reduced. Dynamic ranges of 40 to 20 dB are not uncommon over some part of the frequency spectrum. The threshold of hearing is often more elevated over some parts of the spectrum than others.
2. Description of the Prior Art
Hearing aids have been made in which automatic gain (AGC) with a short attack time and a long release time has been used. This suffers from the disadvantage that vowels are more powerful than consonants while most of the information in speech is contained in the relatively weak consonants. A loud vowel causes the AGC system to reduce gain and the following weak consonant is below the threshold of hearing.
The threshold of hearing is usually not at a constant level with frequency and tone control is needed to enable all weak signals needed to be heard. On the other hand the threshold of discomfort is usually fairly constant with frequency and loud signals should be limited to a fairly constant level at the eardrum e.g. 90 dB SPL for many patients. However, a change of 10 dB with frequency in the level of the threshold of discomfort may occur and, if it rises in a part of the spectrum where the threshold of hearing is also very elevated, it may be advisable to limit sounds 10 dB higher in the raised part of the threshold of discomfort.
Thus, it is desirable to provide a hearing aid system with tone control and signal limitation. However, the system should otherwise deliver the sound without amplitude or frequency distortion. For a patient with binaural capability, an aid on each ear will make speech easier to understand, particularly in conditions of competing noise.
Multi-channel compressor hearing aid systems are known in which the channel amplifiers are set to different compression ratios for each channel. However in such systems, the speech envelope is impaired.